The invention relates generally to establishing a fuel value limit for diesel engines, and more specifically to establishing such a limit based on the ambient conditions in which the diesel engine operates.
Large self-propelled traction vehicles such as locomotives commonly use a diesel engine to drive an electrical transmission system comprising generating means for supplying electric current to a plurality of direct current traction motors whose rotors are drivingly coupled through speed-reducing gearing to the respective axle-wheel sets of the vehicle. The generating means typically comprises a main 3-phase traction alternator whose rotor is mechanically coupled to the output shaft of the engine, typically a 16-cylinder turbo-charged diesel engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase stator windings of the alternator. These voltages are rectified and applied to the armature windings of the traction motors.
During the xe2x80x9cmotoringxe2x80x9d or propulsion mode of operation, a locomotive diesel engine tends to deliver constant power from the traction alternator to the traction motors, depending on the throttle setting and ambient conditions, regardless of locomotive speed. For maximum performance, the electrical power output of the traction alternator must be suitably controlled so that the locomotive utilizes fall engine power. For proper train handling, intermediate power output levels are provided to permit graduation from minimum to fall output. But the traction alternator load on the engine must not exceed the level of power the engine is designed to develop for a given speed. Overloads can cause premature wear, engine stalling or xe2x80x9cbogging,xe2x80x9d or other undesirable effects. Historically, locomotive control systems have been designed so that the operator can select the desired level of traction power, in discrete steps between zero and maximum, so that the traction and auxiliary alternator, driven by the engine, can supply the power demanded by the traction load and the auxiliary loads, respectively.
In the prior art locomotives, when the throttle is advanced from one position to the next (commonly referred to as notches) the diesel engine speed and the load (or excitation) applied to the traction motors are simultaneously increased to the next speed and horsepower point established for the new notch position. The engine acceleration to the new speed point is controlled by the electronic fuel injection controller which adjusts the quantity of pressurized diesel fuel (i.e., fuel oil) injected into each of the engine cylinders so that the actual speed (in rpm) of the crankshaft corresponds to a desired speed. The locomotive control system applies more excitation to the main alternator, which in turn supplies more current to the traction motors, increasing the motor horsepower.
In the prior art locomotive systems, the electronic fuel injection controller acts as the speed governor in response to speed changes requested by the locomotive control system. In the prior art, the speed governor does not receive any signals from the throttle when it is changed from one notch position to another and therefore does not know when a notch change has occurred; the speed governor knows only the speed demand as requested by the locomotive control system. In fact, there are multiple notch settings that vary the horsepower delivered by the traction motors without changing the engine speed.
For each of its eight different notch settings, the engine is capable of developing a corresponding constant amount of horsepower (assuming maximum output torque). When the throttle notch 8 is selected, maximum speed (e.g., 1,050 rpm) and maximum rated gross horsepower (e.g., 4,500) are realized. Under normal conditions, the engine power at each notch equals the power demanded by the electric propulsion system, which is supplied by the engine-driven traction alternator, plus the power consumed by the electrically driven auxiliary equipment.
The electronic fuel injection controller calculates the fuel mass required to maintain the desired engine speed, then converts this value to a pulse duration, through a series of look-up tables. The pulse duration determines the fuel mass that is injected into each cylinder, as measured in mm3/injection. The pulse is input to the pump solenoids that control the injection of fuel into each cylinder. The leading edge of the pulse determines the start of fuel injection, and the pulse length determines the duration during which fuel is injected into the cylinder. The look-up tables provide the required duration of fuel injection (i.e., the pulse duration) as a function of engine speed, speed demand, and start of injection timing. Before the diesel engine is placed in service, the tables are empirically created based on calibration tests performed on a test stand, during which the fuel delivery quantity is measured, while varying fuel injector cam speed (which is a function of engine speed), the start of injection timing, and the pulse duration. The tables are necessarily based on the fuel temperature during the test and the fact that when the test is performed the fuel pumps and injectors are new. Thus, the actual fuel temperature and the fuel pump and injector integrity during the bench test serve as the calibration point for the look-up tables.
It is possible, but not practical, to perform a series of calibration tests at various fuel temperatures and fuel pump and injector conditions. That is, a first calibration test could be performed based on a first fuel temperature and fuel pump and injector conditions based on one year of wear. The second test could be based on the first fuel temperature and a fuel pump and injector condition based on two years of wear. In this way, a series of tables could be created for later use when the diesel engine is placed into service. The appropriate table would be consulted, as a function of fuel temperature and fuel pump/injector wear, to determine the pulse duration. As the fuel temperature changes or the fuel pumps and injectors wear, the appropriate table would be selected from among those available. However, it is well known that it is not practical to create nor store such a large number of tables. Therefore, the prior art has developed certain techniques for determining the fuel value when actual conditions are different from those conditions extant when the calibration tests were conducted. It is known that under changed conditions of fuel temperature and fuel pump and injector wear, a different fuel value (i.e., a different pulse duration) must be commanded to inject the same fuel mass into each cylinder so that the engine speed under the current conditions equals the engine speed during the calibration tests. Another recognized disadvantage of the prior art scheme is the fact that the tables are generic and that one table is used for all engines in the same engine family. Thus subtle variations between individual diesel engines are not accounted for in the fuel tables.
There is a fuel value limit (expressed in mm3/injection) associated with a diesel engine. This limit represents the maximum amount of fuel that can be injected into each cylinder without raising the cylinder pressure above its design value or causing excessive smoke. When the fuel injection controller commands a fuel value at the fuel limit, the diesel engine is derated (i.e., the engine cannot deliver more horsepower) and higher fuel values are prohibited by the controller. For example, assume that a fuel mass of 80 pounds must be injected into each cylinder, per hour, to maintain an engine speed of 1050 rpm at 4500 hp. Using the look-up tables, the electronic fuel injection controller generates a pulse having a duration such that a fuel volume of 1400 mm3 is injected into each cylinder per stroke, to maintain this speed. Now, if the fuel temperature increases by 20xc2x0 F., the fuel density decreases and thus a greater fuel volume must be injected during each injection so that the fuel mass remains unchanged and thus the same engine speed is maintained. The fuel injection controller uses a feedback loop to sense the speed decrease when the fuel density decreases and in response, using the tables, injects a greater fuel volume (i.e., 1415 mm3/injection) to maintain the engine speed at 1050 rpm. Note that the injected fuel volume, as seen by the fuel injection controller, has increased from 1400 mm3/injection to 1415 mm3/injection, but the fuel mass has remained unchanged at 80 pounds per hour. In the prior art fuel systems, the fuel value limit is not changed when the fuel temperature increases, so that in the example above, the headroom between the fuel value and the fuel value limit decreased when the fuel value was increased from 1400 to 1415 mm3/injection. But, in fact, the same mass of fuel was injected in each case.
In the prior art, mechanically operated fuel injection pumps are controlled by cam driven lifters for injecting the fuel through a nozzle into the combustion chamber. The pump is manually set to avoid injecting excessive fuel (i.e., prevent overfueling) into the cylinder by the position of a set screw, which can be adjusted to decrease or increase the amount of fuel injected. Typically, the set screw is adjusted so there is approximately a 1 percent fuel value margin. Assuming a fuel value (volume) of 1300 mm3/stroke for steady-state operation, the fuel value can increase to approximately 1313 mm3/stroke to meet engine load demands. The engine cannot respond to requests to increase speed or horsepower beyond that which can be provided by a fuel value limit of 1313 mm3/stroke. As a result, the engine is derated when this fuel value is reached. Because this prior art fuel value limit system is mechanically controlled, its accuracy is limited by the tolerance associated with the mechanical components, and under certain conditions it unnecessarily limits the fuel value that can be provided to each cylinder without causing overfueling.
In addition to an increase in fuel temperature (which, as discussed above, can be caused by an increase in ambient temperature or an increase in the bulk temperature of the fuel), there are other conditions that require a higher fuel demand per injection to maintain the engine speed. In each case, the amount of energy derived from each fuel injection is the same (as it must be to maintain engine speed) but due to a condition associated with combustion efficiency, a greater quantity (volume) of fuel must be injected (i.e., longer pulse duration) to deliver the same speed and load. If ambient pressure decreases (the locomotive is climbing to a higher altitude), combustion becomes less efficient. Under these conditions, more fuel volume must be delivered during each injection to make up for the lost efficiency and increase in overall fuel consumption and thereby maintain engine speed. A similar decrease in efficiency occurs as the ambient air temperature goes up. Higher ambient air temperature conditions will also require more fuel to be delivered during each injection to maintain the same engine speed.
Another condition that will change the amount of fuel required to maintain engine speed is the amount of energy each fuel injection can provide. In this case, as the amount of energy derivable from a given fuel mass changes, the fuel injection quantity must change to deliver the same total energy to the cylinder and to deliver the same speed and horsepower. If the locomotive takes on a new load of fuel with lower heating value, longer injection durations (providing a greater fuel injection volume) are required to deliver the same amount of energy. The quantity of energy provided by the injected fuel from the new fuel load does not change, but the mass of injected fuel must change so that the energy content can remain the same. Because the energy delivered is unchanged, there is no increase in cylinder pressure. Thus, the fuel value limit, which is established based on the baseline conditions present when the fuel tables were created, does not accurately represent the actual fuel limit that will cause cylinder and engine damage.
Large diesel engines with electronic fuel injection in locomotive applications have considerable flexibility to impact the engine performance through the fuel injection system. For example, when a problem in the cylinder occurs that causes the cylinder to produce low power or no power, the other cylinders can make up for the lost power by the injection of additional fuel into the operating cylinders. Under certain conditions, such as a cold ambient temperature and low altitude, this could drive the operating cylinders into an overfueling condition where each cylinder is running at a higher power than its design capabilities. This higher power increases cylinder pressure, which can then adversely affect the cylinder and other engine components. To avoid the overfueling problem, the fuel injection controller sets a limit on the fuel value that can be delivered to each cylinder. The fuel limit is calculated by the software of the fuel injection controller as a function of engine speed and intake manifold air density. Specifically, in one prior art embodiment, there are two tables from which the fuel value limit is determined. The first table is one-dimensional, listing fuel value limits for each value of engine speed in 50 RPM increments. The second table is two-dimensional where the fuel value limit is a function of both intake manifold air density and engine speed. The intake manifold air density is calculated from the absolute manifold air pressure and manifold temperature. Each speed/manifold air density pair has a corresponding fuel value limit. The two dimensional table is used primarily to control engine smoke in those situations where a lower quantity of air is available. For instance, high altitude, high manifold air temperature, low manifold air pressure, and a transient situation all result in a lower quantity of air available for combustion. The lower fuel value limit between the two tables at any given condition is the limit used by the fuel injection controller. Generally, the results from the one dimensional table are lower than the two-dimensional table during normal steady state operation.
In one embodiment, the maximum fuel value limit from the first table above is set at approximately 6 percent above the standard fuel value. This value represents a compromise between setting a lower fuel value limit that will produce nuisance derating, and recognition of the fact that certain physical and energy conditions of the fuel (as discussed above) raise the fuel value required to meet the speed demand. In one embodiment, the value obtained from the one dimensional table allows the fuel value to increase to a maximum of 6 percent above its nominal value. The 6 percent was chosen to allow for some high altitude, warm ambient and warm fuel operating conditions. A negative effect to a fixed fuel value limit is its inability to effectively deal with the variety of ambient and external conditions to which the locomotive is subjected. For example, when a single cylinder becomes inoperative, the fuel value must be increased by approximately 6 percent to make up for the lost power. The additional fuel injected into each cylinder, up to the point where the 6 percent fuel value limit is reached, can overcome a loss of a single cylinder. (Each cylinder develops approximately 6 percent of the engine horsepower in a 16 cylinder engine.) However, if two cylinders become inoperative, then approximately 12 percent of the engine power has been lost. Since the fuel value limit is only 6 percent over normal, the engine cannot continue to deliver the same power when two cylinders are inoperative (at standard operating conditions). When the fuel limit is reached, the engine controller sends a signal to the locomotive controller to derate the allowable load on the engine, since that load value cannot be provided. As a result, using the 6 percent fuel value limit, it takes a loss of two cylinders to cause a power deration. For optimum engine protection, it may be desirable to derate when only one cylinder is lost but the 6 percent fixed fuel value limit cannot accommodate this. As will be shown below in conjunction with the teachings of the present invention, if ambient conditions are forcing low fuel values, for example, a cold day with operation at low altitude, there may, in fact, be sufficient fuel margin to the fuel limit where overfueling would occur so that the engine could deliver full load with two cylinders inoperable. But the prior art fixed fuel limit does not accommodate this possibility.
The present invention provides a method and system for more effectively, adaptively, and accurately setting the fuel value limit based on ambient conditions, in particular based on the ambient air temperature and pressure, the fuel temperature, the fuel heating value, and the fuel pump and injector wear. By more accurately determining the fuel value limit that will cause engine overfueling based on these operating conditions, the maximum fuel value is raised and the occurrences of engine derating are correspondingly reduced. Overfueling refers to that condition where an excessive amount of fuel is injected into each cylinder, causing the cylinder to run above its rated power. As a result, the cylinder pressure is increased above its rated value, potentially causing adverse affects on the engine and related components. The present invention provides for the determination of a tighter and more accurate fuel value limit, so that overloading the cylinders is a less likely occurrence. Advantageously, adapting the fuel value limit to the actual operating conditions avoids nuisance derating of the locomotive.